In a conventional electric arc melting furnace as shown in FIG. 1 in vertical section, a furnace shell a has a water-cooled roof b at a center of which an electrode c is vertically provided. The furnace shell a has at its bottom an oxygen blasting port d through which oxygen is blasted into the shell a.
The furnace shell a has a spout e at its side. A spout door f outside of the spout e is vertically movable by a door cylinder g such that the spout e is closed and opened by downward and upward moment of the door f, respectively.
The roof b is provided with an exhaust gas duct h which accommodates a conveyer j to charge scrap i into the shell a and which has an air suction port k.
The scrap i is transported together with carboniferous material such as coke by the conveyer j, charged into the shell a and is heated and melted by heat of arc from the electrode c into molten steel 1. The roof b serves to absorb radiant heat from a surface of the molten steel 1.
The oxygen blasted through the port d reacts with the carbon to generate carbon monoxide m which is partially burned with air n intruding into the shell a through the spout e and other gaps. Generally, the quantity of the intruding air n is not sufficient to completely burn all of the carbon monoxide m and therefore about 30-60% of the carbon monoxide m remains unburned and enters into the exhaust duct h in which the carbon monoxide m is re-burned with air from the suction port k. Resultant reaction products to be discharged from the duct h as exhaust gases o preheat the scrap i and carboniferous material transported by the conveyer j into the shell a.
In the conventional electric arc melting furnace shown in FIG. 1 where the carbon monoxide m generated in the shell a is burned with the intruding air n, the quantity of the intruding air n is less as described above and the carbon monoxide m generated is partially burned so that temperature in the shell a is locally increased to generate local heat, disadvantageously resulting in failure of uniform heating of the molten steel 1.
The intruding air n which is relied upon for combustion in the shell a is not controlled at all in relation with the quantity of carbon monoxide m generated in the shell a and the combustion in the shell is left to take its natural course. Therefore, such combustion of the carbon monoxide m cannot be employed for control of temperature in the shell a and, in fact, hardly contributes to any enhancement of thermal efficiency in the shell a depending upon conditions.
Flames are nonuniformly distributed in the shell a due to the partial combustion of the carbon monoxide by the intruding air n so that radiant heat from the surface of the molten steel 1 cannot be effectively absorbed by the water-cooled roof b, disadvantageously resulting in decrease of heat efficiency of the electric arc melting furnace.
The carbon monoxide m, which remains unburned in the shell a in the nonuniform combustion by the intruding air n, is introduced into the duct h with its concentration being left uneven. The introduced carbon monoxide with uneven concentration is nonuniformly or locally burned in the duct h, which may cause the scrap i in the duct h to be nonuniformly preheated or may lead to explosion in the duct h. Preheating temperature of the scrap i cannot be controlled at all.
In view of the above, the present invention has for its object to overcome the above and other defects encountered in the prior art and provide an electric arc melting furnace in which not only combustion temperature in a furnace shell but also preheating temperature in a scrap preheating device are controllable as desired, thereby positively ensuring a high degree of thermal efficiency and safety in furnace operation.